Such an apparatus is known from FR-A-2 519 614. In the known apparatus, the conveyor is carried over two return wheels bearing-mounted in a slide arranged for reciprocation. A first drive unit engages the continuously moving part, and a second drive unit engages the intermittently moving part. The difference in speed of travel between the two parts is set off by displacement of the slide which bearing-mounts the return wheels. A drawback of the known apparatus is the relatively expensive guidance required for bearing-mounting the slide arranged for reciprocation, while the mass of the slide should in each case be accelerated and decelerated, which results in substantial mass forces. In order to limit the occurring mass forces somewhat, the speed of the conveyor is kept relatively low.
Another drawback of the known apparatus is that two drive units are necessary for realizing the continuous and intermittent motion.
Still another drawback of the known apparatus is that the drive elements engaging the conveyor and forming part of the drive units engage the conveyor over only a very small part of the circumference thereof, since both drive elements engage in a straight path of travel. This causes the risk of slip and at any rate, the conveyor will wear quickly on account of this very local engagement. In EP-A-0 390 241, this latter drawback has been overcome by guiding the conveyor along the circumference of the drive element over a larger arc of an angle; however, this has as a consequence that in a conveyor having return wheels accommodated in a slide capable of reciprocation, additional tensioning wheels have to be employed for keeping the tension on the conveyor. This solution also has the drawback that the chain does not move in an entirely convex contour, but follows a concave part at the location of the drive element. In particular when the conveyor is on its convex side provided with article holders such as grippers, a concave part in the contour causes problems, because the grippers must not collide with each other in the concave part. To prevent such collisions, it is necessary that the distance between the grippers is greater than the distance needed when the conveyor moves in an exclusively convex contour. A greater mutual distance between the grippers adversely affects the capacity of the conveyor.